This invention relates generally to riveting, and more particularly concerns improvements in riveting techniques involving expansion of a rivet slug to provide a fastened joint exhibiting high shear strength, high fatigue strength and sealing properties, and many other advantages.
Experience with conventional rivet retention of aircraft skins to fuselage structure has indicated several disadvantages and problems, particularly where fully tapered flush head rivets are employed. It has been found that when the structure including the skin or panel is flexed, the fully tapered hole (countersunk in the skin to receive the head) tends to elongate, i.e. become non-circular, producing a clearance between the rivet head and the tapered seat. As a result, corrosive fluids can and do enter this gap or clearance to corrode the connection over a period of time. Also, "fretting" of the skin material at the countersink frequently occurs, weakening the skin material so that fatigue cracks can and do occur more readily. Paint or other sealant applied over the rivet head and skin surface as a barrier tends to crack in response to flexing and to become ineffective to prevent ingress of corrosive fluid. Also, the paint enters the circular clearance about the rivet head end, to form an unsightly, visible ring. Another problem concerns the relatively large outer diameters of fully tapered conventional rivet heads, which undesirably limits the number of rivets that can be employed in or along a given area or dimension of skin material. A further problem had to do with the difficulty of maintaining a flush relationship of the rivet head and the work surface.